Toll like receptors (TLRs) are pattern recognition receptors present on diverse cell types that recognize specific molecular patterns present in microbes such as bacteria, viruses or fungi (1). TLRs recognize lipoprotein (TLR2), double-stranded RNA (TLR3), lipopolysaccharide (LPS, TLR4), flagellin (TLR5), single-stranded RNA (TLR7/8), and bacterial or viral unmethylated CpG DNA (TLR9). All TLRs, except TLR3, signal through an adapter protein referred to as MyD88, resulting in the activation of NFkappaB and related cytokine genes (2).
TLR7 and 8, located intracellularly in endosome components, recognize single-stranded RNA and a distinct synthetic guanosine analog of host cells (3, 4). Guanine and uridine-rich single-stranded RNA has been identified as a natural ligand for TLR7 (5). In addition, several low molecular weight activators of TLR7 have been identified, including imidazoquinolines, and purine-like molecules (3, 6, 7). Among the latter, 9-benzyl-8-hydroxy-2-(2-methoxyethoxy) adenine (“SM”), has been identified as a potent and specific TLR7 agonist (8). Derivatives of SM have been synthesized by incorporating an aldehyde functional group on the benzyl moiety and coupling that intermediate to different auxiliary chemical entities through a bifunctional linker molecule containing a hydrazine and N-hydroxysuccinimide (9). Conjugation to mouse serum albumin (MSA) protein increased potency by 10 to 100-fold and improved the in vivo pharmacodynamics compared with the free drug. The MSA conjugate could be delivered to the respiratory system by intranasal or intratracheal administration. Drug delivery by intranasal proved to be effective in two mouse models of infectious disease, a bacterial infection and a viral infection (9). The SM intermediate also has been conjugated to a lipid, dioleoylphosphatidyl ethanolamine (DOPE), and it was determined that the conjugate possessed enhanced TLR7 agonist activity (e.g., WO 2008/115319, published on Sep. 25, 2008, based on International patent application no. PCT/US2008/001631 filed Feb. 7, 2008).